Effect of Dietary Protein and Energy Levels on Rumen Fermentation in Holstein Steers

Two experiments using six rumen-fistulated Holstein steers in 3 × 3 Latin square designs were conducted to test the effects of urea addition to corn silage, and grain addition to haylage, on feed intake, digestibility of nutrients and rumen fermentation of complete diets fed once daily. Addition of .34 and .68% urea to corn silage increased dietary crude protein from 8.1 to 11.3 and 13.3%, and stimulated dry matter intake 6 and 13%. No change in dry matter or acid-detergent fiber digestibility was noted among treatments. Rumen contents, sampled at 2 h intervals for 24 h postfeeding, exhibited two distinct peaks in ammonia (NH3) concentration at 2 and 12 h. Rumen pH decreased through 12 h and then increased. Total VFA increased from 7 to 11 mmol/dl between 0 and 14 h and then decreased. Individual VFA's followed a similar pattern. Added nitrogen (N) increased (P<.05) concentrations of acetic, butyric and total VFA. Little difference in molar percentages of VFA was noted among treatments and sampling times, suggesting that N stimulated the amount of VFA produced rather than the type of rumen fermentation. Addition of corn to alfalfa haylage diets at 30 and 60% of dry matter (DM) stimulated dry matter intakes 19 and 38%, respectively, and increased DM, N and acid-detergent fiber (ADF) digestibilities about 8%. However, total ADF disappearance from the digestive tract was 25% lower on 60% corn, compared with 0 and 30%. Diurnal trends for rumen NH3, pH and VFA were similar to corn silage diets, but changes were less rapid. Grain addition decreased (P<.05) pH (.5 to .7 units) and increased concentrations of VFA (from 30% for acetic to 100% for butyric). As with corn silage, grain substituted for up to 60% of DM in haylage diets increased amount but not proportions of VFA.